Inductive, conductivity sensors serve in a large number of applications in laboratory and process measurements technology for registering the conductivity of a liquid, measured medium. They are preferably used where large measuring ranges and high chemical or thermal loadings occur. This is true, for example, in a large number of industrial, chemical processes, however, also for hot steam sterilization methods, which are frequently applied in the foods technology field due to its associated high requirements as regards hygiene.
An inductive, conductivity sensor includes a transmitting coil and a receiving coil, which are, as a rule, embodied as ring coils, which are also referred to as toroidal coils. Such a conductivity sensor functions as a kind of double transformer, wherein the transmitting and receiving coils are inserted sufficiently far into the measured medium that a closed electrical current path can form extending through the measured medium and passing through the transmitting and receiving coils. When the transmitting coil is excited with an alternating voltage signal, it produces a magnetic field, which induces in the closed path through the medium and through the coils an electrical current, whose level depends on the electrical conductivity of the measured medium. Since this alternating electrical current in the medium brings about, in turn, a variable magnetic field surrounding it, an alternating electrical current is induced in the receiver coil. This alternating electrical current, respectively a corresponding alternating voltage, delivered by the receiver coil as output signal is a measure for the electrical conductivity of the measured medium.
For supplying the transmitting coil with an alternating voltage, an inductive, conductivity sensor includes a driver circuit connected with the transmitting coil. For registering the output signal of the receiving coil, the conductivity sensor includes, moreover, electrically connected with the receiving coil, a receiving circuit, which is embodied to output the registered measurement signal (which may, in given cases, be conditioned by the receiving circuit) to a sensor electronics, which serves to process the measurement signal further and, in given cases, to digitize it. Frequently, conductivity sensors are embodied as measuring probes at least sectionally immersible in the measured medium. Such a measuring probe includes a measuring probe housing, in which are accommodated the transmitting and receiving coils, and, in given cases, the driver circuit and the receiving circuit, as well as other circuit parts assembled with the transmitting and receiving circuit in a sensor circuit. The measuring probe is connected in such an embodiment with a remotely situated, superordinated unit, for example, a display unit, a measurement transmitter, or a computer. The superordinated unit can be embodied both for energy supply of the measuring probe as well as also for data communication with the measuring probe. The sensor circuit contained optionally in the measuring probe can be embodied to forward the further processed, in given cases, digitized, measurement signal to the superordinated unit. The corresponding measured value can be displayed via the superordinated unit by means of a display system or output via a data interface.
Inductive, conductivity sensors of this type are known, for example, from U.S. Pat. No. 3,603,873, DE 198 51 146 A1, DE 41 16 468 A1, DE 10 2006 025 194 A1 as well as DE 10 2006 056 174 A1.
Housings of inductive, conductivity sensors are manufactured, guided by the above-described principles of operation, such that the housing surrounding the transmitting and receiving coils does not form a closed, electrically conductive path. Therefore, preferably non-conductive materials are utilized for the housing. The coils of inductive, conductivity sensors can, in such case, be provided in different ways with a non-conductive housing formed, most often, of a plastic material. A known construction of an inductive, conductivity sensor includes, for example, a coil support body of metal, in which the transmitting and receiving coils are arranged, and which is surrounded with a non-conductive synthetic material, for example, with injection molded polyetheretherketone (PEEK). The coil support body serves to protect the coils against high injection pressures and high temperatures during injection molding and, in the subsequent operation of the sensor, to shield the transmitting and receiving coils against undesired direct couplings between the coils.
Described in German patent, DE 10 2010 042 832 A1 is an inductive, conductivity sensor having a plastic housing, which contains a coil component. The coil component includes a support plate, on whose front side the transmitting coil and on whose rear side the receiving coil are arranged coaxially relative to the rotational symmetry axes of the transmitting and receiving coils. The support plate includes an opening, which is aligned with oppositely lying openings of the plastic housing. A plastic sleeve, whose ends are connected sealed against the medium by ultrasonically welded connections with the openings of the plastic housing, is led through the opening of the support plate and passes also through the transmitting coil and the receiving coil. If the housing is immersed in a measured medium, the measured medium also passes through the sleeve and forms, thus, a path of medium passing through the coils, so that, such as above described, when the transmitting coil is excited with an alternating voltage, an electrical current flow is induced extending along a closed path through the measured medium. The welded connection between the sleeve ends and the openings of the housing means that the housing is sealed to media, so that the measured medium cannot penetrate into the housing and into the circuits, respectively the sensor electronics, contained in the housing.
The plastics used for housings of inductive, conductivity sensors must meet diverse requirements as regards workability, as well as chemical, mechanical, thermal, also optical properties. Moreover, plastics coming in contact with the measured medium must, depending on the character of the application, in which the sensor is to be applied, fulfill certain requirements as regards biocompatibility or, especially in the fields of the foods industry and the pharmaceuticals industry, be qualified for certain permits as regards their physiological compatibility, permits which must be granted, for example, by health officials. Thus, the scope of plastics suitable for the manufacture of the housing can be limited. Especially, there are no plastics, which have all the desired properties of the housing, so that for the material selection lastly a compromise must be explored. Thus, for example, the application of glass fiber reinforced plastics enables the cost effective manufacture of mechanically very stable housings, yet these materials are, most often, not allowable for use in the foods field. In the foods field, therefore, sensors of unreinforced, permitted materials, such as PEEK or perfluoroalkoxy polymers (PEA), must be applied, which have a lesser mechanical stability.